A sequence is called an arithmetic progression, if the difference of two consecutive term is the same always.
i.e. an – an–1 = d, (constant), n ∈ N.
Here d is called the common difference (If d= 0 sequence is a constant sequence. if d>0 the sequence is increasing; if d<0,the sequence is decreasing)
nth term of an A.P = a+(n–1)d
nth term from end of an A.P = an + (n – 1) (–d)
where an is the last term and d, the common difference of the A.P.